Interpretive Summary: The prion diseases are a group of fatal brain disorders of sheep, goats, cattle, deer and elk. An abnormally folded protein accumulates in some lymphoid tissues of sheep early in disease. Biopsy sampling of lymphoid tissue, including tissue in the rectum, is a suitable live animal test in sheep. Adaptation of that test for use in deer exposed to the cervid prion disease Chronic Wasting Disease has been proposed. In this paper, the investigators compared the results of testing rectal tissue with test results on brain and the lymphoid tissues currently used for early diagnosis of the disease. Deer from a captive farm with a high prevalence of disease and wild deer with a low prevalence of disease were included in the study. Nearly eighty percent of the deer with abnormal prions in lymphoid tissue or brain had detectable abnormal prion proteins in the rectal lymphoid tissues. Although lymphoid tissues of the head remain the tissue of choice for early diagnosis of the disease in deer, the use of rectal lymphoid tissue is a suitable adjunct, particularly for live-screening farmed deer at risk for chronic wasting disease.

Technical Abstract:
The transmissible spongiform encephalopathies are a family of fatal neurodegenerative diseases characterized by accumulation of abnormal prion proteins in the brain. The abnormal prion protein is the major constituent of the infectious agent and is a reliable marker for disease. The occurrence of a zoonotic prion disease in cattle has resulted in efforts to eradicate or control all prion diseases in domestic livestock, including scrapie of sheep and chronic wasting disease CWD of deer and elk. Antemortem testing of sheep, deer and elk is based on the finding that abnormal prion proteins accumulate in some lymphoid tissues months or years before being detectable in brain. Biopsy of tonsil is a suitable test for live deer but requires general anesthesia. Biopsy sampling of the recto-anal mucosal associated lymphoid tissue (RAMALT) has been suggested as an alternative site for antemortem testing in sheep. In this study, postmortem sampling of RAMALT tissue from deer was performed to estimate the diagnostic sensitivity and specificity of the test. Samples were assayed by monoclonal antibody based immunohistochemistry and the results of RAMALT testing were compared with testing of brain, tonsil and retropharyngeal lymph node, the currently preferred tissue for early diagnosis. Sensitivity of the test was 80% in a sample of 76 white tailed deer from a captive facility and 77% in a sample of 210 free ranging white tailed deer. While the retropharyngeal lymph node remains the tissue of choice for early diagnostic testing, RAMALT biopsy may provide a suitable adjunct, particularly for antemortem testing of herds of farmed deer with potential exposure to the disease.